<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        li {
            position: relative;
        }

        li div {
            position: absolute;
            width: 400px;
            height: 400px;
            background: red;
            top: 0px;
            left: 50px;
            display: none;
        }

        li:hover div {
            display: block;
        }
    </style>
</head>

<body>
    <ul>
        <li>
            <a href="">手机</a>
            <div></div>
        </li>
    </ul>
</body>

</html>